Recently, in consideration of environmental issues, an electric powered vehicle such as an electric vehicle, a hybrid vehicle or a fuel cell vehicle has attracting attention. Such an electric powered vehicle has a rechargeable power storage unit mounted thereon, to supply electric power to an electric motor, and to convert kinetic energy to electric energy for storage at the time of regenerative braking.
Conventionally, a vehicle charge system for charging the power storage unit mounted on such an electric powered vehicle using an external power supply, for example, commercial power supply, has been known. For instance, Japanese Patent Laying-Open No. 08-107606 discloses a charge device for an electric powered vehicle, including charge priority control means for inhibiting running state of the electric powered vehicle even when a main switch is on, as long as a charging voltage is being supplied from a charger, and for continuing charging of a battery. By the charge device for an electric powered vehicle, charging is given priority even when the main switch is operated by mistake.
In these days, charging of a power storage unit mounted on a hybrid vehicle by an external power supply, for the purpose of improving general fuel consumption efficiency, has attracting attention.
In order to supply electric power necessary for generating vehicle driving force, a power storage unit having relatively high voltage is used as the object of charging. For efficient charging of the power storage unit having high voltage, optimization of charging current and charging voltage is desirable. Therefore, it has become a common practice to mount a power converting device on such an electric powered vehicle, for generating charging voltage and charging current that are suitable for the power storage unit, from an external power supply.
The power converting device is formed of a switching element such as a transistor. Therefore, power conversion by the power converting device requires a control signal (for example, gate voltage or base current) to cause the switching operation of the transistor. In most cases, such a control signal is generated by a controller operating with low voltage power from a low voltage power storage unit having relatively low output voltage of, for example, 12V or 24V, provided separate from the power storage unit as the object of charging.
Therefore, once the low voltage power storage unit enters the over-discharged state, that is, when the battery “goes flat” it becomes impossible to activate the controller even if external power is supplied to the electric powered vehicle. As a result, the power converting operation by the power converting device cannot be executed, and the charging by the external power supply becomes impossible.
As a solution to such a problem, it may be possible to lower the voltage of high voltage power storage unit using the voltage converting device and to supply the voltage to the low voltage power storage unit that has “gone flat.” A control signal for controlling the voltage converting device, however, must also be generated by the controller that operates with the low voltage power from the low voltage power storage unit. Therefore, the problem remains unsolved.